1. Field of the Invention
The present invention relates to a method of purifying an alkaline solution by effectively nonionizing or removing metallic impurity ions in the alkaline solution and a method of etching semiconductor wafers which can etch them without deteriorating the quality of the semiconductor wafers using a purified alkaline solution purified by the purifying method.
2. Description of the Related Art
Generally, a method of manufacturing semiconductor wafers includes a slicing step for obtaining wafers of thin disk shape by slicing a single crystal ingot; a chamfering step for chamfering a peripheral edge portion of the wafer obtained through the slicing step to prevent cracking and/or breakage of the wafer; a lapping step for flattening the surface of the chamfered wafer; an etching step for removing processing damages of the chamfered and lapped wafer; a polishing step for mirror-polishing the surface of the etched wafer; and a cleaning step for cleaning the polished wafer to remove a polishing agent and/or particles attached thereon.
The etching step may involve an acid etching process using an acid etching solution or an alkaline etching process using an alkaline etching solution such as sodium hydroxide or the like.
The acid etching is characterized by its high etching rate, so that it is difficult to uniformly etch over the entire surface of a wafer, thus causing a problem that the flatness of the wafer is reduced. For this reason, recently, an alkaline etching, which uses a sodium hydroxide solution, a potassium hydroxide solution, and an alkyl ammonium hydroxide solution, and so on, is predominantly employed because the alkaline etching does not deteriorate the flatness of the wafer.
In the alkaline etching of semiconductor wafers mentioned above, an industrial alkaline solution available on the market, having a rather high metallic impurity concentration, is used as it is. Even an alkaline solution of electronic industrial grade used for etching semiconductor wafers actually contains metallic impurities of several tens of ppb to several ppm.
Metallic impurities included in the alkaline solution may be nickel, chromium, iron, copper and so on.
It has been revealed the fact that when a semiconductor wafer is etched using an alkaline solution containing such metallic impurities, metallic ions of some of metallic impurities such as copper, nickel or the like dissolved in the alkaline etching solution diffuse deeply into the inside of the wafer, during the alkaline etching, to cause a deteriorated wafer quality so that the characteristics of semiconductor devices formed by the wafer are significantly degraded.
As measures to prevent a deteriorated wafer quality due to the alkaline etching solution as mentioned above, the use of a highly pure alkaline solution may be contemplated. However, such highly pure alkaline solutions available on the market are only extremely expensive ones of analysis grade, and the use of such an expensive alkaline solution for industrial use is not at all feasible in terms of cost. It has been found, in addition, that even these highly pure alkaline solutions are not sufficient for preventing a deteriorated wafer quality.
It is contemplated that to solve the problems of the prior art mentioned above, an alkaline solution should be more purified. Generally, it tends to think that the purification of a solution requires to remove contaminant metal impurities from the solution of interest. However, according to a recent outcome of the present inventors' devoted researches, it was revealed that the mechanism of contaminating a silicon substrate with metallic impurities contained in an alkaline solution is caused by deposition of metallic ions existing in the alkaline solution as dissolved species on the surface of the silicon substrate by adsorption or electro-chemical reaction.
It was found from this fact that an effect similar to substantial purification of an alkaline solution can be produced by transforming metallic impurities into a form which does not cause the contamination, even without removing metallic impurities from an alkaline solution. In other words, if impurity metallic ions existing in an alkaline solution are nonionized, the quality of semiconductor wafers will not be deteriorated even if the wafers are etched using an alkaline solution physically including metallic impurities (fine solid metal impurities) therein. Thus, the present invention was completed.